Pregnant Sprague-Dawley rats treated with 0, 2.0 or 6.0 mg/kg of methylmercury on days 6-9 of gestation or left untreated as part of the Collaborative Behavioral Teratology Study (CBTS) were assigned to either the CBTS or Cincinnati Test protocol after birth. Offspring assigned to the Cincinnati test system were evaluated for growth, mortality, incisor eruption, eye opening, vaginal patency, surface righting, negative geotaxis, pivoting, olfactory orientation, swimming ontogeny, figure-8 activity, and complex water maze (Biel) problem solving. Methylmercury lengthened gestation, reduced maternal weight, and increased offspring preweaning mortality at the higher dose. This dose also accelerated upper and lower incisor eruption and delayed vaginal patency development. The high dose produced a non-significant reduction in offspring weight from shortly after birth to 30 days of age, and a significant reduction in weight by 60 days of age. This dose caused a significant delay in surface righting development and swimming ontogeny, while the low dose accelerated negative geotaxis turning and swimming angle development. The high dose reduced postweaning figure-8 activity, increased Biel water maze time, errors, and proportion of trial failures (no escape within 6 min), although the effect on errors was not significant. It was concluded that at the doses and exposure period used here, methylmercury was confirmed to be a potent behavioral teratogen using the Cincinnati test system. This finding is in agreement with the results obtained with the same treatment regimen in the CBTS. Two tests from the Cincinnati test system, swimming ontogeny and Biel maze, provided evidence that they would significantly improve the detection power of the CBTS test battery.